The present invention relates generally to a cathode for an electron gun and its manufacturing method, and more particularly to a cathode for an electron gun of which thermoelectron emission characteristics is enhanced and of which durability is prolonged and to a method thereof especially adapted for manufacturing it.
Conventional cathode for an electron gun of a cathode ray tube, as shown in FIG. 1, comprises normally a cylindrical-type sleeve 1, a nickel base metal 2 containing a small amount of silicon, magnesium and the like and capping the top portion of the sleeve 1, a thermoelectron emission substance layer 3 composed of alkaline earth metal carbonate such as barium (expressed as Ba), strontium (Sr) and calcium (Ca) and deposited on the base metal 2, and a heating element 4 provided in the sleeve 1.
In the above-described cathode for an electron gun, the thermoelectron emission substance layer will be typically manufacturing by the following process.
First, a compound is prepared by mixing the powder of carbonate of alkaline earth metal such as barium, strontium and calcium with binder and organic solvent such as isoamyl acetate, n-butanol, butyl acetate and the like, and then a suspension is prepred by dispersing the compound through a ball mill. Here, emission paste(it is referred to as EP hereinafter) as thermoelectron emission substance is obtained by mixing the suspension for a period of 24 hours. The EP is deposited on the above-described base metal through spraying process so as to form a thermoelectron emission substance layer.
The thermoelectron emission substance layer formed by the above process is changed into composite oxide through an aging step of the cathode ray tube manufacturing process.
In case where the thermoelectron emission substance is composed of carbonate of alkaline earth metal such as barium, strontium and calcium, that will be changed into the triple composite oxide of barium, strontium and calcium by the following chemical formula. ##STR1##
The composite oxide formed by the above process is heated to about 900.degree..about.1100.degree. C. once again, and through the heating process, the composite oxide is reacted by the following chemical formula by means of reducing agents such as silicon and magnesium contained in the base metal, and thus some part of composite oxide will have the characteristics of semiconductor. EQU 2BaO+Si.fwdarw.2Ba+SiO.sub.2 EQU BaO+Mg.fwdarw.Ba+MgO
In the above chemical formula, it has been found that a part of BaO contained in the compound of alkaline earth metal is deoxidized to generate free barium under high temperature atmosphere, thereby effecting the thermoelectron emission.
However, the cathode manufactured by the above-described process has problems as follows.
1) An intermediate resistance layer composed of Ba.sub.2 SiO.sub.4 or the like is formed in an interface between the base metal and the thermoelectron emission substance layer due to the result of the deoxidization reaction during an activation process, and therefore the intermediate resistance layer will interrupt the flow of electric current.
2) Since the intermediate resistance layer is present and continues to grow, the composite oxide of alkaline earth metal and the reducible element are restrained from reacting together, thus suppressing the generation of free barium.
3) The thermoelectron emission substance layer and the intermediate resistance layer are oxide layers having a low electric conductivity, so that, if they are forced to generate a large quantity of thermoelectrons, the joule heat is excessively generated by the electric resistance, having thermoelectron emission substance rapidly consumed. That will result in the shortening of the durability of the electron gun.
For solving the above-mentioned problems, there have been disclosed cathode manufacturing methods in Japanese laid open patent publication No. 61-269828 and No. 61-271732.
In these methods, scandium oxide (expressed as Sc.sub.2 O.sub.3) treated with heating at a temperature of from 800.degree. to 1100.degree. C. under atmospheric pressure for 30 minutes to 2 hours is mixed with EP of thermoelectron emission substance in the ratio of 0.1.about.20% by the weight of scandium oxide to EP. That will be capable of manufacturing a cathode having electric current density 2 A/cm.sup.2 and 30,000 hours in life time.
However, the above-described cathode manufacturing method using the additive Sc.sub.2 O.sub.3 has the following problems due to adding Sc.sub.2 O.sub.3 as fourth substance in powder state to EP.
1) Since the scandium oxide is added to EP not only in powder state but also in extremely small amount in comparison with that of EP, it is very difficult to have the scandium oxide uniformly distributed into the thermoelectron emission substance layer provided on the base metal even though using EP sufficiently dispersed.
2) For that reason, scandium oxide in powder state may be locally concentrated on the thermoelectron emission substance layer provided on the base metal, and thus that will form a portion of the scandium oxide into composite oxide together with barium, strontium and calcium. That will cause the nonuniform distribution of electric conductivity to occur, thereby to bring about the unbalance in the thermoelectron emission characteristics.
3) Accordingly, for enhancing the dispersion state of the scandium oxide, a large amount of the scandium oxide more than that actually used should be consumed, thereby increasing in production cost.